A Pressure switch is a commonly used instrument in the process industry.  Like many instruments used in science, they need to be calibrated to ensure their accuracy and reliability.  The wrong calibration can cause errors in results or damages the equipment.  This can cost a laboratory or industry a significant amount of money.

What is a Pressure Switch and How Does it Work?

A pressure switch is an instrument that measures pressure and has an electrical switch function programmed to operate at a certain pressure.

It can be set so that when there is no pressure connected, the switch is closed.  When pressure increases, the switch opens.

Normally Open (NO) and Normally Closed (NC) Switches

Normally open switches have the switch terminal open when no pressure is connected.  When enough pressure is applied, the switch closes.  Normally closed switches are closed when no pressure is connected and when enough pressure is applied it opens.  The determining factor in which one is best for your use is dependent on what kind of circuit you want to drive with the switch.

Dry and Wet Switches

A dry switch has the connections being open or closed, so it is working like a mechanical switch.  A wet switch has two different voltage values representing the two output states.

The output of an electrical wet switch can be a voltage signal with two levels, a current signal, or an open collector type signal.

Safety Pressure Switches

Safety switches are used in the safety instrument systems, and they have specific safety classifications.  The calibrations of these are regulated.

These switches stay static most of the time without ever functioning.  They don’t toggle open and closed in normal usage; they are waiting if the safety alarm level is met then they operate.  Because these switches do not operate a ton, there is the risk that they will not work when needed.

When calibrating do not exercise these safety switches prior to calibration, instead of capture the very first point when the switch operates.  It may be that the first operation that requires more pressure than the operations after a few exercises.  Regular switches are typically exercised a few times before calibration.  In a safety switch, the operation point is critical.

Considerations in Selecting a Switch Type

When selecting a switch type, the state should be considered so that should the power supply fail, or a cable becomes loose, the switch status should remain safe.  In the case of a safety switch, it should be configured so that in case a cable comes loose, the alarm goes on.  It is designed to be fail-safe.

How to Calibrate Pressure Switches

There are a few steps when getting ready to calibrate switches.

• Preparation & Safety – If the switch is installed in the process, it is important to make sure that it is isolated from the pressure line. You also need to make sure to disconnect any circuit that the switch is controlling.  Some switches may have main voltage, or another dangerous voltage, across the switch terminals when they open, so make sure that it is isolated.
• Pressure Ramp – To calibrate a pressure switch you need to provide a slowly changing pressure ramp, moving across the operating points of the switch. Depending on the switch type, you need to first supply a suitable pressure to start the calibration.  There are a few ways to provide the input pressure, a calibration hand pump, shop air supply with a precise pressure controller, or an automatic pressure calibrator.  It is vital to provide a slow pressure ramp.  So that you can see the precise pressure whereby the switch operates.  If the pressure changes too quickly, you cannot accurately capture the pressure point when the switch operates.
• Measuring the Switch Output – You will need a tool to measure the switch terminals. In some cases, it may be an Ohm meter, a voltage meter, or a current meter.  Some tools have the option to program a trigger level that suits the switch and enable the status change to be captured automatically.
• Capturing the Operation Points – In the switch calibration, you need to capture the input pressure at the very moment when the output state changes. Some devices can capture the input pressure automatically at the very same moment when the switch output changes its state.  However, you can capture the input pressure manually, only with a very slow input pressure for accuracy.
• Delayed Output – Some industrial switches may have a delay added to the output so that it does not work too quickly. You should be able to find out if your switch has a delay.  As the calibration needs to be done slower than normal.

SRP Can Help

SRP control systems ltd offers solutions and help for pressure switch calibration and many other types of instrument calibration.  Contact us today and let us make your business run as smoothly as possible.